A standard self-tightening hammer-drill chuck that is rotated about an axis of a drill spindle to rotate a drill bit about and reciprocate it along the axis has a chuck body securable to the spindle and formed centered on the axis with a screwthread. A sleeve rotatable but axially nondisplaceable on the chuck body is formed angularly equispaced about the axis with a plurality of angled jaw guides, although it is possible to form the guides on the chuck body and the screwthread on the sleeve for the same effect. Respective jaws in the guides have racks that mesh with the screwthread so that rotation of the sleeve body on the chuck body about the axis in a tightening direction moves the jaws radially together and opposite rotation in a loosening direction moves the jaws radially apart. The normal clockwise (seen from behind the tool) rotation of the drill chuck creates reaction forces in the chuck which tend to rotate the sleeve in the tightening direction for automatic self-tightening action.
As described in my copending patent application Ser. No. 07/429,843 filed Oct. 31, 1989 (now U.S. Pat. No. 4,955,623) a locking ring is provided that is limitedly angularly displaceable relative to the chuck body. In addition this ring is formed with teeth or other formations that mesh axially with complementary formations on the sleeve and the ring is axially displaceable on the chuck body between a back unlocked position with the formations disengaged and relative rotation of the ring and sleeve possible, and a front position with the formations in mesh and such relative rotation impossible. The function of this ring is to prevent accidental opening of the chuck. The limited angular movement of the ring on the chuck body is necessary to allow the above-described self-tightening action to take place and is created by providing the ring or chuck body with a pin projecting radially into a slot on the other body that is of somewhat greater angular width than the pin.
With such an arrangement when a drill bit is to be chucked the locking ring is first pulled back, and then the sleeve is rotated in the unlocking direction, normally counter-clockwise as seen from axially behind the chuck, to spread the jaws sufficiently to accept the tool. Then the sleeve is rotated in the locking direction until the jaws snugly engage the shank of the tool, and finally the locking ring is pushed back forward so that its teeth engage those of the sleeve and impede it from rotating substantially on the chuck body.
In order to avoid that the locking ring is in its angular end position corresponding to engagement of its formation in the locking direction with the chuck body or vice versa, that is in such a position that it completely impedes further tightening of the chuck I provide in the system of my above-cited copending patent application a projection that is fixed in and extends radially from the chuck or locking ring toward the other body. The other body is formed with an angularly extending slot having angularly opposed and spaced end surfaces spaced apart angularly by a distance greater than the angular dimension of the projection and axially opposed and spaced side surfaces spaced axially by a distance generally equal to the axial dimension of the projection. Thus when the projection is engaged in the slot same limits relative angular displacement of the bodies. An axially backwardly extending cutout opens axially forward into the slot adjacent one of the end surfaces thereof at a mouth having an angular dimension generally equal to the axial dimension of the projection. Thus the cutout only allows axial travel of the locking ring into its locked position when the projection is aligned axially with the cutout at the one end surface of the slot.
With such a system the chuck can indeed self tighten during drilling. Nonetheless afterward it is possible for it to loosen again.